Wide-angle objective lens system and camera

ABSTRACT

A vehicle camera and wide-angle objective lens system are disclosed wherein the wide-angle objective lens has image aberrations or errors that do not deteriorate the detection of obstructions or obstacles in its field of view.

BACKGROUND OF THE INVENTION

The invention relates to a wide-angle objective lens system and awide-angle camera incorporating the wide-angle lens system.

Today, in the case of motor vehicles, especially in regard to commercialvehicles, more and more cameras are being installed for the purpose ofmonitoring the area surrounding the vehicle. Panoramic cameras of typesto serve such a purpose are rigidly affixed to a vehicle so that thecamera has to encompass a wide area of sight or a wide-angled field ofview. For this purpose cameras with comparatively inexpensive wide-angleobjective lens systems are customarily installed.

These wide-angle objective lenses encompass a very wide field angle of100° or more. However, this large field angle, i.e., this extensiveextent of view, must be bought at the price of considerable distortionand reduced brightness in the outer edges of the displayed image.“Distortion” or “image distortion,” means that straight lines in theedge areas of the object show themselves as curved in the producedimage. It is important to distinguish a barrel-shaped distortion and acushion-shaped distortion. Wide-angle lens system usually show abarrel-shaped distortion, i.e., expressed as positive figure. FIG. 3illustrates examples of positive and negative image distortions. Imagedistortion is expressed as

$\left( \frac{y^{\prime} - y}{y^{\prime}} \right),$where y′ represents the height of the image without distortion and ydesignates the height of the image with aberration. As a general rule,only the radial distortion, i. e. the change in lengths along radialdistances, are considered. In the present application %-values fordistortion designate this radial image distortion.

Where objective lenses are involved, and especially wide-angle objectivelenses, a multiplicity of errors or aberrations in the produced imagesoccur, which are known as the “Seven Seidel Aberrations” referring tothe Seidel Error Theory. These seven Seidel Aberrations can be combinedinto three groups:

-   -   I) focus aberrations        -   a) Spherical aberration, (differences in paraxial rays and            marginal rays),        -   b) coma, (light patching, with stringed tails),        -   c) astigmatism, (formation of focal lines at points),    -   II) positional aberrations        -   d) field curvature, (result of curved surface of image            receiver),        -   e) distortion, (convex or concave forms of outlines or            barrel-shaped or cushion shaped outlines),    -   III) color aberration        -   f) longitudinal color aberration        -   g) transverse color aberration

Each lens of an objective lens system possesses various properties suchas the kind of glass, curvature expressed by the radii of the two lenssurfaces) and the thickness of the lens. The arrangement of a pluralityof lenses in an objective system becomes characterized by the separationdistance of one lens from another, the position of an iris, and the backfocus, i.e. the distance of the last lens surface to the plane of therecorded image. These characteristics become known as parameters ordegrees of freedom. Theoretically, each of these degrees of freedom canbe put to use to correct image aberrations. Contrary thereto, eachdegree of freedom takes part in all image aberrations. By customary useof optics software the proportionate image aberration for each singlelens surface can be calculated.

In the following the work method of an optics designer will be explainedby the aid of a pertinent example. This example is very important, sinceit presents the concept, of how an optics-designer proceeds and it showsdecisive the creativity of the optics designer still is. It is possibleto correct the seven aberrations with a minimum of eight independentsystem parameters. Focal length is also such a parameter. A triplet,i.e., a three-lens objective, could, as far as principle is concerned,suffice in this correction. A triplet is normally built-up from twoconverging outside members, e. g. made of crown glass, and one innerdiverging member, e. g. made of flint glass. This assembly provides sixradii and two distances between the individual lenses. To start with,the optics designer brings together optical system parameters such asthe type of the glass, the thickness of the lenses, the separatingdistance between the lenses and also the radius of curvature of theglass surfaces. We have six lens surfaces and it is now possible todetermine to what extent each contributes to the overall aberration inthe final image. Very simplified, we could determine, that in a givencase the radius of the second surface of the first lens produces aspherical and chromatic aberration, and the radii of the third lenssurfaces produce coma and astigmatism.

The optics designer must now make a decision, as to how theseaberrations are to be corrected. He may try to change the curvature ofthe first lens in order to correct for the spherical aberration.However, the curvature of the lens is also decisive for the focal lengthand the focal length should not be changed. The change of the curvaturemay reduce the spherical aberration but at the same time coma would beincreased. The designer can also decide that the correction is to bedistributed over a plurality of system parameters, in order toameliorate the erroneous sensitivity. If a specific parameter is verydecisive in order to correct for a certain aberration, one is introuble, if the parameter is outside the allowed tolerance or clearancerange during the production of the lens system. Or, one can alsodetermine, that the clearance is too finely specified, and cannot beobserved in the production of the lens system.

The optics designer will alter the system parameters up to such a point,that the remaining optical aberrations are small enough. In additionalsteps, he will attempt to correct each image aberration with differentdegrees of freedom simultaneously. The burden of the correction can thenbe distributed over the various lens surfaces and the entire system isno longer as critical. Within certain limitations, the optics designerhas the possibility of specifying the types of glass and the degrees ofcurvature, although each chosen combination brings forth another aspectof the total correction. If the triplet has been so configured, that itapproximates the pre-defined requirements, then the designer can, forexample, determine that the astigmatism at the edge of the image hasnearly vanished, but appears to play an important role in the innerfield of view. At this point, we collide with a new problem. The sevenSeidel aberrations, outlined above, are, unfortunately, not the onlyoptical aberrations. One designates the Aberrations of Seidel as “Imageaberrations of the Third Order”. Logically, there are more aberrationsof a higher order. The most important of these are the aberrations ofthe fifth and seventh order. These aberration groups are generally onlyto be encountered when the first group, the “third order” aberrations isproperly corrected.

Theoretically, a very small point existing in the object is mapped intoa very small point again. As a matter of practice, this does not occurbecause of the optical aberrations. A point will not reproduce as apoint, but rather as a small disk with a varying distribution ofbrightness. As soon as these disks under-step a certain diameter, thenthe image errors become evident. That is a very simplified explanation.In reality, these aberrations are continually in force, but come toattention only if the residual aberrations of the third order are small.

The example given, i.e. the “Triplet Example”, wherein the astigmatismin the field is still visible, shows the effect of these imageaberrations of higher orders. One can make use of a defined and entirelycontrolled residue of the Seidel image aberrations in order tocompensate for errors or aberrations of the fifth and seventh order.This is naturally, a limited measure and a triplet will have only anacceptable image quality, if the field angle and/or the iris opening issmall.

It is important to note that a defined optical system, defined by thenumber and the configuration of the lenses etc., provides for limitedcorrection possibilities. That means, in other words, even withsophisticated optics software and computer power only an experiencedoptics designer will choose the “right” starting parameters.

Computers, software and numerical methods are used in order to reduceoptical aberrations. This equipment and procedures are be employed inorder to optimize an optical system. This huge amount of data may causeits own problems. As a result, the task of the experts or opticsdesigners has not become easier. Rather with the aid of computers anoptics designer can consider more parameters and carry out thecomputations quicker and with greater accuracy.

A certain relationship exists between the number of design parameters ofan optical system (lens curvature, lens thickness, distance ofseparation, refractive index etc.) and the degree of correction of theoptical aberrations. With a greater degree of freedom and more designparameters, respectively, the optics designer has correspondingly morepossibilities of correcting a system. If an optics designer applies agreater number of optical elements, then a better degree of correctioncan be attained. This, however, results in a considerable increase incosts, and further, the system may react strongly on the part ofmanufacturing clearances or increases in weight.

The designer of an optical system must then acquire a very goodunderstanding of the fundamental optical possibilities of a givenconstruction. All constructions or designs require an optimizationsystem or plan in accord with a initial sketch. If the construction isnot suitable for a fine compensation of aberrations, then the opticsdesigner will attain only a product of lower quality.

A six lens objective system has 10 free lens surfaces (radii), six lensthicknesses (one per lens) with four separation-distances between thelenses. Additionally, each kind of glass has its own refractive indexand dispersion number to consider. Further, it is necessary, that theexact position of the iris is to be determined. With these 36parameters, i.e., degrees of freedom, the optics designer must correctmore than 60 different image aberrations. Each parameter can presentsomething like 10,000 individual values and one must calculate some6,000 different ray paths for each parameter change.

These 36 degrees of freedom or parameters are also not entirelyindependent. Some must be combined; others are strongly limited by otherparameters. Accordingly, the 36 degrees of freedom are reduced tosomething like 20, whereby the task becomes even more complex. In viewof the given conditions and considerations, it is not surprising, thathundreds, if not thousands of designs may result, all of which veryclose or similar to the desired solution or design. The completeevaluation of a six lens objective system with the aid of fast computersand software that are able to calculate 10.000 lens surfaces per secondapproximately takes ten years.

Obviously, such a procedure is not feasible. In order to seek out thebest solution to this unending succession of choices, the opticsdesigner must have an inherent recognition of all the effects of theimage aberrations on the final image quality of the displayed image. Inaddition, he must have the capability, to know those factors of imagequality, which can produce the desired features of the optical system.

In a case of the application of wide angle objective lenses for thepanoramic viewing of the immediate environment about a vehicle, thereshould be, first, the ability to encompass the greatest possible fieldof view, since the cameras are normally affixed rigidly to the vehicle.Second, the image aberrations, which will necessarily appear, must notdeteriorate the recognition of obstacles within the field of view of thewide-angle lens system. Moreover, a wide-angle objective lens systemcannot be designed in too complex a manner, since then it would be tooexpensive for use in a motor vehicle.

Thus it is an object of the present invention to make available aneconomical, wide-angle objective lens having image aberrations or errorsthat do not deteriorate the detection of obstructions or obstacles inits field of view. It is a further object o the present invention toprovide a camera with such a wide-angle lens system.

SUMMARY OF THE INVENTION

Because the eventual placement of the wide-angle lens and,correspondingly, the wide-angle camera is on the outside of a vehicle,it is necessary that wide-angle cameras must be provided with aprotective cover. In order not to change the optical characteristics andproperties of the optical imaging system of the wide-angle camera theseprotective covers are conventionally designed with a shell or dome likecontour. The costs for such shell like contours or dome shapedprotective covers are high, particularly when one considers, that theoptical properties of the protective cover must not interfere withoptical properties of the wide-angle lens to be protected.

Because, in accordance with the invention, the protective cover consistsof a flat, transparent plate, this formation is essentially cheaper.However, the flat plate must be considered in the calculation of thewide-angle lens as an included optical element. For this reason,surprisingly, the total costs for this addition to the present inventionturns out to be less than corresponding costs using the conventionalshell shaped protective cover.

Preferably, this protective cover has parallel surfaces on each flatside, since the optical effect of a parallel plane plate permits itselfto be more convenient in the computation of the objective lens.

Glass adapts itself excellently as a material for the protective plate,since glass possesses excellent optical properties and, further, glassis very resistant to the environment. Since the diameter of the platelies in the general range of 5 cm, the known breakage characteristics ofthe glass plays no role.

It has been recognized herein that an essential problem, which ariseswith the installation of wide-angle objective lenses in motor vehicleapplications, is the image distortions which are inherent in suchlenses, i.e., image distortions deteriorate a quick and easy recognitionof obstructions within the field of view. Thus, according to theinvention with a wide-angle lens system having a diagonal field of viewof 118° or 120° the distortion is reduced by the wide-angle lens systemitself to <10% and preferably <5%. With a wide-angle lens system havinga diagonal field of view of 142° the distortion is reduced by thewide-angle lens system itself to <17% and preferably <15%. Although theautomotive installation scope of the present invention excludes, becauseof costs, the installation of complex and refined wide-angle lenssystems, nevertheless, the correction of the distortion is carried outby means of appropriate design of the lens system and not throughelectronic data processing of the captured images. Surprisingly,experience has shown that achieving this is possible at acceptable lowcosts.

According to an aspect of the invention, a reduction of image distortionis mainly achieved by means of using an aspherical lens. Preferablyaspherical lens is concave-convex bi-aspherical in design, and the lensis positioned as the last lens in the wide-angle objective systemadjacent to the image sensor, that is, just before the focal plane orimage receiving plane of the image sensor.

According to a further aspect of the invention the optical assemblylength or geometric length of the objective system is limited to 18 mm±5mm for a wide-angle lens system having a diagonal field of view of118°/120°, and to 21 mm±5 mm for a wide-angle lens system having adiagonal field of view of 142°. This forcibly requires first, a simpledesign and second, yields a wide-angle objective lens system, which iscompact enough for installation in a motor vehicle.

According to a further aspect of the invention the wide-angle objectivelens is comprised of five lenses at the most, whereby the firstlens-group consists of three lenses at the most and the secondlens-group consists of two lenses at the most. This number of lensesoffers a satisfactory compromise between complexity of the objectivelens and thus its price and adequate possibilities to correct imageaberrations.

According to a further aspect of the invention, with a wide-angle lenssystem having a diagonal field of view of 118°/120° the iris, measuredfrom the entrance aperture, is positioned at 60%±10% of the overalloptical installation length of the lens system. With a wide-angle lenssystem having a diagonal field of view of 142° the iris is positioned at75%±10% of the overall optical installation length of the lens system.Experience has shown that this location for the iris is particularlyadvantageous in reducing evolved aberrations and particularlydiminishing image distortions, considering the small number of thelenses which are present.

According to a further aspect of the invention, the lenses of the firstand the second lens-group are placed in direct contact with one another,without any spacer element or the like. In this way, an especiallysatisfactory length of assembly is gained, i.e., the overall dimensionsof the lens system are reduced. Also clearances are maintained in abetter manner, since the separating distances do not exist.

According to a further aspect of the invention, the optical iris has adiameter of 1.26 mm±0.5 mm in a wide-angle lens system having a diagonalfield of view of 118°/120°, and a diameter of 0.85 mm±0.5 mm, with awide-angle lens system having a diagonal field of view of 142°. Thereduction of image distortion is particularly enhanced by thisdimensioning.

According to a further aspect of the invention, the first lens-groupcomprises three lenses and the second lens-group encompasses two lenses.This combination of individual lenses, their positioning, anddimensioning has the effect of bringing about a very large field of viewand at the same time creates a very small degree of distortion. The restof the image aberrations hold themselves within tolerable limits.

According to a further aspect of the invention, the dimensioning of thefive lenses provides satisfactory results in regard to the reduction ofdistortion and other optical characteristics of the wide-angle objectivelens system in accord with the present invention.

According to a further aspect of the invention, the iris is created witha circular opening in a cylindrical boring. By this means, disturbingreflections due to grazing incident light in conically bored irisopenings are avoided.

According to a further aspect of the invention, a flat, transparent,front protective plate is provided for the objective lens, instead of aconventional spherical or domed front cover. This creates the necessity,that the optical characteristics of the said plate must be givenconsideration in the computation of the wide-angle objective lenssystem. The increased costs associated therewith, however, are more thancompensated for by the lesser expense of the flat plate in comparison tocosts of the mentioned spherical or domed lens cover.

According to a further aspect of the invention, the first lens of thesecond lens-group is especially designed for the correction of fieldcurvature or image bulging. By means of these corrections the spatiallycurved image surface of the wide-angle objective lens is adapted to thatthe plane surface of the image sensor. In other words, image aberrationsare thereby corrected and minimize

DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood from a reading of thefollowing specification and by reference to the accompanying drawingsforming a part thereof, wherein an example of the invention is shown andwherein:

FIG. 1 a sectional drawing of an embodiment example of a wide-anglecamera with an electronic image taking unit system in accord with thepresent invention;

FIG. 2 an optical, function diagram of the embodiment example of FIG. 1with a presentation of the path of rays from various angles of the fieldof exposure; and

FIG. 3 a schematic presentation of distortion, depicted as one of theSeidel Aberrations.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1, 2 show an exemplary embodiment of the invention. FIG. 1illustrates a wide-angle camera with a wide-angle objective lens systemin accord with the present invention depicted in a longitudinal sectionalong the optical axis of the objective. FIG. 2 presents the path ofrays through the wide-angle objective system in accord with FIG. 1,shown embracing various view angles designated as α and α/2,respectively.

Light from a target object (on the left side of FIGS. 1 and 2, notshown) converges from a field angle α to enter a first lens-group 2,which has an entrance aperture 4. Following the direction of incidentlight from the object to be taken, i.e., for example, to bephotographed, behind the first lens-group 2 is located an optical iris6. In turn, behind the optical iris 6 is provided a second lens-group 8.Following the second lens-group 8, adjacent a flat image surface orfocal plane 10, is located an electronic image capture unit in the formof a CCD-sensor 12 with a plurality of pixels. Between CCD-sensor 12 andsecond lens-group 8 is located an IR-cut filter 14 (IR=Infra Red). IRrays are filtered-out through the said IR-cut filter 14, as these wouldlead to a degradation of the quality of the image. This is especiallytrue for the presentation of colour in the case of colour cameras, sincewhere a high proportion of IR rays are present, the color effect isdiminished in quality.

The first lens-group 2 includes a first lens 16-1, a second lens 16-2and a third lens 16-3. The second lens-group 8 consists of a fourth lens16-4 and a fifth lens 16-5. The lenses 16-1, 16-2 and 16-3 of the firstlens-group 2 are placed in contact with one another, i.e., no separatingdistance or spacer element is provided between them. Likewise, the forthlens 16-4 and the fifth lens 16-5 are also contiguous in the samemanner. In this case, the larger fifth lens 16-5 acts as a holder forthe smaller fourth lens 16-4. All five lenses 16-1 to 16-5 are retainedin place by a lens retainer 18. Retainer 18 grips or contacts the fivelenses 16-1 to 16-5 at their radial rims. The left side opening of thelens retainer 18 also defines the size of the entrance aperture 4.

The optical iris 6 is positioned immediately in front of the fourth lens16-4. The iris 6 is formed by the shape of the lens retainer 18 in thespace between the third and the fourth lenses 16-3, 16-4. The iris 6 isformed in a part of the lens retainer 18 extending across the opticalaxis OA and comprises a blind boring 20 having a cylindrical shapeforming an entrance aperture 22 of the iris 6. At the bottom ofcylindrical blind boring 20 is a second boring 23 forming an exit port24 allowing light to pass through the iris arrangement. The diameter ofblind boring 20 is larger than the diameter of second boring 23. Blindboring 20 and second boring 23 are designed to symmetrically encirclethe optical axis OA of the objective lens system in lens retainer 18.Exit port 24 is placed immediately in front of fourth lens 16-4 and itsdiameter determines the size or diameter of iris 6. By means of thecylindrical shape of the boring 20 instead of the common cone shape, thetransmission of grazing light through iris 6, which would lead toundesirable reflections, is avoided.

In the following the specific data and decisive optical parameters fortwo exemplary embodiments of the invention are given. The structure oftwo embodiments is very similar so that FIGS. 1 and 2 arerepresentations of both embodiments.

The first embodiment of the invention has a field of view α of 118° or120°. In the first embodiment the first lens 16-1 is convex-concave inshape and possesses radii of R₁₁ and R₁₂ for the curvatures of therespective lens surfaces, as may be seen in Table 1 which follows. Thesecond lens 16-2 is likewise convex-concave with a first radius R₂₁, anda second radius R₂₂. The third lens is biconvex with a first radius R₃₁and a second radius R₃₂. The fourth lens is also biconvex shaped withrespective radii designated R₄₁, R₄₂. The fifth lens is aconcave-convex, aspherical lens. The thicknesses, diameters, andrefractive indices of the five lenses 16-1, -2, -3, -4, -5 as well astheir separation distances are shown in the following table 1:

TABLE 1 refraction thickness/ no. symbol radii/mm mm nd/587 nmdiameter/mm first lens R11 +11.60 1.50 1.79 16.00 16-1 R12 −5.20 airspace 3.00 second lens R21 +45.20 0.80 1.77 11.00 16-2 R22 −6.10 airspace 1.60 third lens R31 +44.90 2.90 1.84 9.50 16-3 R32 +10.90 airspace 4.00 Iris 0.04 1.26 fourth lens R41 +4.30 1.40 1.77 4.00 16-4 R42+7.80 air space 0.90 fifth lens R51 −0.53 1.00 1.53 2.40 16-5 R52 +1.102.90

The diameter for the fifth lens 16-5 given in table 3 is the effectiveoptical diameter.

The aspheric coefficients “c” and the conical constants K of the fifthlens 16-5 of the first embodiment are given in the following table 2:

TABLE 2 aspherical coefficients Conical Radii C2 C4 C6 C8 constant K R510.65470 0.06311 0.08367 0.03340 −0.84600 R52 0.02660 0.03690 0.010900.01770 −0.50240

The circular entrance aperture 4 has a diameter of 14 mm and the iris 6has a diameter of 1.26 mm.

A second embodiment of the invention has a field of view a of 142°.

In the second embodiment the first lens 16-1 is convex-concave in shapeand possesses radii of R₁₁ and R₁₂ for the curvatures of the respectivelens surfaces, as may be seen in Table 3 below. The second lens 16-2 islikewise convex-concave with a first radius R₂₁, and a second radiusR₂₂. The third lens is piano-convex with a first radius R₃₁ and a secondradius R₃₂. The fourth lens is also biconvex shaped with respectiveradii designated R₄₁, R₄₂. The fifth lens is a concave-convex,aspherical lens. The thicknesses, diameters, and refractive indices ofthe five lenses 16-I as well as their separation distances are shown inthe following table 3:

TABLE 3 refraction thickness/ no. symbol radii/mm mm nd/587 nmdiameter/mm first lens R11 +13.73 0.84 1.77 16.,00 16-1 R12 −4.75 airspace 3.11 second lens R21 +153.44 0.72 1.79 11.,00 16-2 R22 −6.04 airspace 1.62 third lens R31 ∞ 3.66 1.85 9.50 16-3 R32 +8.54 air space 3.74Iris 0.01 0.85 fourth lens R41 +6.80 1.24 1.75 4.00 16-4 R42 +4.22 airspace 0.83 fifth lens R51 −0.42 1.24 1.53 2.10 16-5 R52 +0.59 2.72

The diameter for the fifth lens 16-5 given in table 3 is the effectiveoptical diameter. The diameter of the fifth lens 16-5 asthree-dimensional object is 7 mm.

The aspheric coefficients “c” and the conical constants K of the fifthlens 16-5 of the second embodiment can be seen in the following table 4:

TABLE 4 aspherical coefficients conical Radii C2 C4 C6 C8 constant K R510.92546 0.17160 0.22119 −0.00761 −0.88956 R52 0.32994 0.18753 −0.035890.06766 −0.83027

In the second embodiment the circular entrance aperture 4 has a diameterof 15.6 mm and the iris 6 has a diameter of 0.85 mm.

As to the mathematical representation of the aspherical surfaces of thefifth lens 16-5, reference is made for both embodiments to the textbook:Naumann/Schröder, Bauelement der Optik, Taschenbuch for TechnischenOptik, Vol. 5, published 1987, pages 145ff.

In both embodiments the fifth lens 16-5 is composed of plastic and isconstructed as one piece. The fifth lens 16-5 comprises a lens component26 and a holding means 28. The lens component 26 comprises anaspherical, concave-convex lens, which provides for the optical functionof the fifth lens 16-5. The holding means 28 extends itself circularlyaway from the lens component 26, whereby, in a sectional view (seeFIG. 1) said holding part 28 consists of two T-shaped elements, whichextend—see the FIG. 1—from the rim of the lens component 26, in bothupward and downward directions. Accordingly, the holding means 28encompasses a first section 28-1 of circular, annular shape with arectangular cross-section, which connects itself directly onto the rimof the lens component 26. Attached to outside of the first section 28-1is cylindrical second section 28-2 of rectangular cross section. Thesecond section 28-2 is arranged transversely to the first section 28-1.The second section 28-2 of the holding means 28 abuts against the lenssupporting structure 18. The annular first section 28-1 serves as asupporting surface for the fourth lens 16-4. The fourth lens 16-4 isthus in direct contact with the fifth lens 16-5 without a spacer elementthere between and the fourth lens 16-4 abuts against the first annularsection 28-1 of the fifth lens 16-5. That part of the lens retainer 18,which encircles the exit port 24 of the iris 6, holds and supports thefourth lens 16-4. Furthermore, the first, second and third lenses, 16-1,16-2 and 16-3 mutually support each other at their rim circumferencesand are further supported in the radial direction by the lens holder orlens retainer 18.

Immediately in front of the first lens 16-1 a transparent protectivecovering in the form of a parallel-surfaced, plate 30 is arranged. Plate30 guards the wide-angle objective lens from influences of theenvironment. The provision of the parallel-surfaced, transparent plate30, instead of the usual means for conventional cameras, namelyspherical or dome shaped front covers, introduces the condition that theoptical characteristics of the plate 30 must be taken into considerationin the computation of the wide-angle lens. This additional expense,however, is fully compensated for by the essentially lower costs of theparallel surfaced plate 30, when its expense is compared to that of thementioned spherical protective transparent cover.

FIG. 2 presents an optical function diagram of the embodiments shown inFIG. 1 and shows the arrangement of the five lenses 16-1 to 16-5 alongthe optical axis OA of the wide-angle objective lens system in accordwith the present invention. In FIG. 2 the optical constructive length ofthe wide-angle lens system, i.e., the distance between the forward edgeof the first lens 16-1 and the image surface of the image sensor 12 orfocal plane 10 is depicted. In the case of the first embodiment of theinvention, the optical constructive length is 18 mm in case of thesecond embodiment the optical construction length is 21.4 mm.

In FIG. 2 the path of rays 32-1 to 32-5 for five different field of viewangles α is shown. The maximum field of view angle α is represented bypath ray 32-1. For the sake of clarity, in FIG. 2 there is shown onlythe symbol for α/2, this being the angle between the optical axis OA andthe respective incident ray path 32-i. Additionally, in FIG. 2 thethickness and the radial extent of the individual lenses 16-1 to 16-5are depicted.

While a preferred embodiment of the invention has been described usingspecific terms, such description is for illustrative purposes only, andit is to be understood that changes and variations may be made withoutdeparting from the spirit or scope of the following claims.

1. A wide-angle objective lens system comprising: a first lens grouphaving a plurality of lenses; an entrance aperture for the gathering oflight from an object over a diagonal field angle α of 118°±10°; anoptical iris arranged behind said first lens group; and a second lensgroup arranged behind said optical iris, wherein said second lens groupprojects light from the first lens group which has passed through saidiris to impinge on an image surface with an image distortion of <5%, andwherein said optical iris is located at 60%±10% of the opticalconstruction length (OBL) relative to the entrance aperture.
 2. Thesystem of claim 1 wherein said optical iris has a diameter of 1.26mm±0.5 mm.
 3. The system of claim 1 wherein said first lens groupincludes a first, second and third lens and said second lens groupcomprises a fourth and a fifth lens; said first lens is convex-concavewith a first radius of 11.6 mm±δ% and a second radius of −5.2 mm±δ%;said second lens is convex-concave with a first radius of 45.2 mm±δ% anda second radius of −6.1 mm±δ%; said third lens is biconvex with a firstradius of 44.9 mm±δ% and a second radius of 10.9 mm±δ%; said fourth lensis biconvex with a first radius of 4.3 mm±δ% and a second radius of 7.8mm±δ%; said fifth lens is concave-convex biosphere; and ±δ% lies in arange between 1% and 15%.
 4. The system of claim 3 wherein said firstlens has a refractive index of about 1.79.
 5. The system of claim 3wherein said second lens has a index of refraction of about 1.77.
 6. Thesystem of claim 3 wherein said third lens has a refraction of about1.84.
 7. The system of claim 3 wherein said fourth lens has a refractionof about 1.77.
 8. The system of claim 3 wherein said fifth lens has arefraction of about 1.53.
 9. The system of claim 3 wherein said fourthlens is designed to correct the distortion or aberrational swelling orcurvature of the image.
 10. The system of claim 1 further comprising: alens holder wherein said optical iris includes a cylindrical boringformed in said lens holder, wherein said optical iris has an irisopening in a bottom of said cylindrical boring, and wherein saiddiameter of said cylindrical boring is larger than that of said opticaliris.
 11. A wide-angle objective lens system, comprising: a first lensgroup having an entrance aperture for the gathering of light from anobject over a diagonal field angle a of 142°±10°; an optical irisarranged behind said first lens group; a second lens group arrangedbehind said optical iris; and an image surface, wherein said secondgroup projects light from said first lens group which has passed throughsaid optical iris to impinge upon said image surface with an imagedistortion of <17%, and wherein an optical constructed length (OBL) ofsaid wide-angle objective lens measures 21.4 mm±5 mm.
 12. A wide-angleobjective lens system, comprising: a first lens group having an entranceaperture for the gathering of light from an object over a diagonal fieldangle a of 142°±10°; an optical iris arranged behind said first lensgroup; a second lens group arranged behind said optical iris; and animage surface, wherein said second group projects light from said firstlens group which has passed through said optical iris to impinge uponsaid image surface with an image distortion of <17%, and wherein saidoptical iris is placed at 75%±10% of the optical construction length(OBL) relative to the entrance aperture.
 13. The system of claim 12wherein said optical iris has a diameter of 0.85 mm±0.5 mm.
 14. Thesystem of claim 12 wherein said first lens group comprises a first,second and third lens and said second lens group comprises a fourth anda fifth lens, said first lens is convex-concave with a first radius of13.7 mm±δ% and a second radius of −4.7 mm±δ%; said second lens isconvex-concave with a first radius of 153.4 mm±δ% and a second radius of−6.0 mm±δ%; said third lens is plano-convex with a first radius of ∞ andwith a second radius of 8.5 mm±δ%; said fourth lens is biconvex with afirst radius of 6.8 mm±δ% and a second radius of 4.2 mm±δ%; said fifthlens is concave-convex biosphere; and δ% lies in a range between 1% and15%.
 15. The system of claim 14 wherein said first lens has a refractiveindex of about 1.77.
 16. The system of claim 14 wherein said second lenshas a refraction index of about 1.79.
 17. The system of claim 14 whereinsaid third lens has a refraction index of about 1.85.
 18. The system ofclaim 14 wherein said fourth lens has a refraction index of about 1.75.19. The system of claim 14 wherein said fifth lens has a refractionindex of about 1.53.
 20. The system of claim 14 wherein said fourth lensis designed to correct a distortion or aberrational swelling orcurvature of the image.
 21. The system of claim 12, further comprising:a lens holder, wherein said optical iris includes a cylindrical boringformed in said lens holder, wherein said optical iris has an irisopening in a bottom of said cylindrical boring, and wherein saiddiameter of said cylindrical boring is larger than that of said opticaliris.
 22. The system of claim 12, further comprising: a transparentprotective plate disposed before said entrance aperture.
 23. The systemof claim 22 wherein said protective plate is a parallel surfaced plate.24. The system of claim 22 wherein said plate is a glass plate.